An organic EL device is being earnestly studied since it is a complete solid state device and can form a display and an illumination of light-weight and thin-form that is driven at a low voltage.
A problem on applying the organic EL device to a display is to make the driving voltage being a lower voltage. For example, by using a dendrimer of an aromatic amine disclosed in JP-A-4-308688 as a hole injection material, the driving voltage is lowered. The compound has a small ionization potential of 5.2 eV owing to the phenylenediamine skeleton, and exhibits an effect of lowering the driving voltage.
However, the compound having a phenylenediamine skeleton has a small hole mobility of 3×10−5 cm2/V·s, and therefore the driving voltage in the region of high electric current injection is insufficiently lowered.
A high molecular weight aromatic amine compound disclosed in JP-A-9-301934 has a small ionization potential of 5.2 eV but has a problem in that the hole mobility is insufficient. It is expected that the hole mobility be lowered due to the mixing of impurities.
That is, in the fluorescent spectrum of the compound disclosed in JP-A-9-301934 (FIG. 1), a light emission component having a maximum fluorescent wavelength peak of 500 nm or higher is observed, which is not present originally. This shows that impurities are mixed. Furthermore, the voltage is increased by 2.7 V with only driving time of 76 hours, which becomes a hindrance of lowering the driving voltage. Accordingly, in the device disclosed in the publication, the hole mobility is lowered and the driving voltage is increased by the impurities.
Furthermore, because it has a green fluorescent component, when the compound is used in the hole transporting zone of a blue light emitting device, blue light emission cannot be obtained due to mixing of green light emission component.
International Patent Publication WO98/30071 (published on Jul. 9, 1998) discloses an organic electroluminescence device using a compound similar to the present invention, but fails to disclose an effect in that a particular low voltage can be obtained on combining with a light emitting layer containing a charge injection auxiliary.